As sensors become increasingly small, there is a need to produce imaging optics on a comparably small scale. The traditional approach to building lens assemblies for small scale imaging optics is to stack lens elements in a lens cell with spacers and an aperture. The alignment of the individual lenses is entirely dependent on the lens fabrication tolerances and the mechanical tolerances of the lens cell and spacers.
De-center and tilt of the lens elements are the largest contributors to performance degradation in small scale imaging objectives. As lens assemblies shrink in size, the mechanical tolerances used to fabricate the lens and lens cell do not, making the impact of the tolerances on image quality more substantial. As the size of the lenses become sufficiently small, the system becomes inefficient to manufacture because of a high yield loss.
FIG. 1 is illustrative of the shortcomings found in prior imaging optics implementing several lens elements. Prior imaging optics assemblies are made of individual lenses stacked in a lens cell with mechanical spacers. For these systems, the final lens element of the stack is typically held in place by applying epoxy between the edge of the lens and the inside surface of the lens cell. The alignment of the lenses is determined by the mechanical tolerances of the lens cell, the mechanical tolerances of the spacers and fabrication tolerances of the lenses themselves. As the size of the lenses, lens cells and spacers becomes smaller, at some point the fabrication tolerances can no longer be reduced. At such small sizes the error in each mechanical dimension becomes a greater percentage of overall dimension than previously and the potential for misalignment increases, as described below.
A simplified lens cell 100 is shown, which can be a lens cell which is used in any small optic application, having a window element 102 on one end and an image capturing device 104 on the other end. Only a few lens elements are shown for exemplary purposes, but the number of lens elements can extend from one end of the lens cell 100 to the other. The arrows to the left of the window element 102 are representative of light waves entering lens cell 100. As an example, five lens elements are shown, 106, 107, 108, 109 and 110 in other examples more lens elements could be included. An aperture and spacers could also be added. Spacers 112, 113, 114 and 115 are placed between the lens elements to maintain stability and provide structure for convex surfaces. Since lens cell 100 is of a small diameter, in the range of less than 1 mm, some of the lens elements 106, 107, 108, 109 and 110 cannot be manufactured to exacting standards. The inability to manufacture lens elements to exacting standards results in lens elements 110 being the incorrect size and having its center misaligned with the centers of lens elements 107, 108 and 109. Lens 110 is too small for lens cell 100, as seen by the space remaining above lens 110. Lens 106 is also indicative of a manufacturing defect and is tilting at an incorrect angle, as seen at the top and bottom of lens cell 100 where the outer edges of lens 106 are not sitting flush with the interior of lens cell 100. This tilting and misalignment results in loss of image quality.
What is desired is a small scale imaging objective which can be manufactured efficiently, does not depend on the mechanical tolerances of individual lens elements and will provide good image quality.